1. Field of the Invention
This invention relates in general to an image pick-up tube having a photoconductive image pick-up tube and more particularly it provides an image pick-up apparatus in which the signal to noise ratio is substantially improved over prior art devices and wherein a first stage circuit of a pre-amplifier is mounted on the face plate of the image pick-up tube so as to reduce the lead lengths relative to prior art devices.
2. Description of the Prior Art
The signal current that is obtained from a photoconductive image pick-up tube such as a Plumbicon (.TM.) or a Saticon (.TM.) is generally very small and is of the order of 0.3 .mu.A even for the white portion of the projected image. Thus, with the image pick-up tubes using photoconductive image pick-ups, it is necessary to amplify the image output signal from the image pick-up tube before subjecting the image output signal to various signal processing. Also, it has been well known, that with the pre-amplifier for amplifying the image output signal from the image pick-up tube that the first stage circuit is the substantial influence on the signal to noise ratio of the entire system. In order to improve the signal to noise ratio, it is necessary to increase the input resistance and also reduce the stray capacitance due to the electrodes and the wiring lead length of the image pick-up tube.
FIG. 1 illustrates the general construction of a pre-amplifier. An image pick-up tube 1 is provided with a signal electrode ring which is connected to a first stage 3 which is a feedback type circuit and uses an amplifier element which is a junction type field effect transistor 4 that has a high neutral conductance. The output of a subsequent amplifier stage 9 receives the output of the field effect transistor 4 and is fed back through a feedback resistor 5 to the gate of the field effect transistor 4. As illustrated in FIG. 1, a resistor 6 is provided to apply a target voltage V.sub.TT through the signal electrode ring to the image pick-up tube 1. A capacitor 7 is connected in series for blocking DC current. The resistor 6 and capacitor 7 may be omitted where the target voltage V.sub.TT is maintained at ground potential with the application of a negative voltage to the cathode side of the image pick-up tube 1.
In the pre-amplifier as described above, the reduction of the SN ratio due to the input resistance will not occur if the feedback resistor 5 has a value of more than 1 M.OMEGA.. Previously, however, the pre-amplifier was formed on a printed circuit board and was separately mounted from the image pick-up tube and was connected to the image pick-up tube by leads. The stray capacitance C.sub.0 in the first stage circuit 3 is increased by the lead lengths particularly the grounding leads and, thus, the signal to noise ratio is reduced due to the stray capacitance C.sub.0 which results in an inferior device in operation.